Major changes:
* A data structure to collect locked addresses in memory.
* Operations to lock and unlock addresses.
* Remove [ctree_Forall] and express it using [Forall] and [ctree_flatten]. This
saves a lot of lines of code.
* Add a [void] value. This value cannot be typed, but will be used as a dummy
return value for functions with return type [void].
Minor changes:
* Various deciders in preparation of the executable semantics.
* Improve naming and notations.
* Remove obsolete stuff.

The refactoring includes:
* Use infix notations for the various list relations
* More consistent naming
* Put lemmas on one line whenever possible
* Change proofs into one-liners when possible
* Make better use of the "Implicit Types" command
* Improve the order of the list module by placing all definitions at the start,
then the proofs, and finally the tactics.
Besides, there is some new machinery for proofs by reflection on lists. It is
used for a decision procedure for permutations and list containment.

Both the operational and axiomatic semantics are extended with sequence points
and a permission system based on fractional permissions. In order to achieve
this, the memory model has been completely revised, and is now built on top
of an abstract interface for permissions.
Apart from these changed, the library on lists and sets has been heavily
extended, and minor changed have been made to other parts of the prelude.

Most interestingly:
* Use [lia] instead of [omega] everywhere
* More many generic lemmas on the memory to the theory on finite maps.
* Many additional list lemmas.
* A new interface for a monad for collections, which is now also used by the
collection tactics.
* Provide an additional finite collection implementation using unordered lists
without duplicates removed. This implementation forms a monad (just the list
monad in disguise).

The following things have been changed in this revision:
* We now give a small step semantics for expressions. The denotational semantics
only works for side-effect free expressions.
* Dynamically allocated memory through alloc and free is now supported.
* The following expressions are added: assignment, function call, unary
operators, conditional, alloc, and free.
* Some customary induction schemes for expressions are proven.
* The axiomatic semantics (and its interpretation) have been changed in order
to deal with non-deterministic expressions.
* We have added inversion schemes based on small inversions for the operational
semantics. Inversions using these schemes are much faster.
* We improved the statement preservation proof of the operational semantics.
* We now use a variant of SsReflect's [by] and [done], instead of Coq's [now]
and [easy]. The [done] tactic is much faster as it does not perform
inversions.
* Add theory, definitions and notations on vectors.
* Separate theory on contexts.
* Change [Arguments] declarations to ensure better unfolding.